Users that don’t like to carry their smartphone (e.g. while exercising) and need voice calls may want the voice/data SIM for their LTE smartwatch.
Also this voice/data SIM in the LTE smartwatch is a convenient backup phone for those who need to have two phones for reliability (on-call, VIP, etc.).

Activate data-only

You must order a free data-only SIM to get the free shared data

Type the code on the SIM card attachment into a web browser

Power OFF the watch, and using the spanner tool that comes with the LTE watch, remove the AT&T or other SIM

A Wear OS watch will function
semi-independently
AS LONG AS your smartphone is on and internet connected somewhere in the world using get LTE or WiFi for emails/phone texts/et al.
E.g. you forgot your smartphone at home on the charger–you will get emails etc. on your Wear OS smartwatch over Wifi or LTE.

Google Fi says that devices compatible with T-Mobile will generally
work with Google Fi.
I notice that the device is on T-mobile normally and may roam to AT&T (not guaranteed to roam, per Google Fi terms of service).

Activate Phone/Text/Data

Google Fi / Google Voice will
NOT number forward
to another Fi device. Maybe a third-party number forwarder would allow one phone number to ring watch and phone.

You must order a VOICE/TEXT SIM, not a data SIM for the watch (see data-only section if you don’t want phone on the watch)

Data Metering

The Google Fi app will break out the usage for each data-only SIM separately, so you can see what the LTE smartwatch vs. tablet vs. phone is using.